Various techniques are currently employed for bonding fiber reinforced plastic (FRP) members together for use in automotive body applications such as, but not limited to, hoods, doors, bumpers, and the like. There has been an outgrowth in the number of apparatuses and methods available for producing bonded FRP assemblies due to the growing trend in the automotive industry to replace heavier metal components with plastic assemblies. These assemblies are typically bonded by heating an adhesive placed between the mating surfaces of two FRP members to a temperature exceeding its curing temperature.
Prior art FIGS. 1-3 illustrate in a simplified manner, examples of well-known bonding techniques that use heat to bond an FRP assembly consisting of FRP members 10 and 12 with adhesive 14 placed there between. FIG. 1 illustrates dielectric heating apparatus 16 that produces radio frequency electrostatic fields between electrode 18 and block member 20. The electrostatic fields quickly heat adhesive 14 to a temperature above its curing temperature to thereby bond FRP members 10 and 12. Commonly assigned U.S. Pat. No. 4,941,936 to Wilkinson et al. and U.S. Pat. No. 4,941,937 to Iseler et al. discloses examples of dielectric heating techniques which are hereby incorporated herein by reference. Dielectric heating techniques have the advantage of reducing cycle times along with the accompanying disadvantage of heating an FRP assembly in a manner that is difficult to control and maintain.
Prior art FIG. 2 illustrates bonding apparatus 22 that utilizes cartridge heaters 28 for heating metal block members 24 and 26. Block members 24 and 26 in turn heat the air flowing through air circuits 30, a portion of which flows through openings 32 for heating adhesive 14 between FRP members 10 and 12. One disadvantage with this type of bonding technique is that it requires a large supply of compressed air to operate efficiently.
Prior art FIG. 3 illustrates bonding apparatus 34 that utilizes heated steam and/or hot water flowing through passages 40 for heating metal block members 36 and 38. Block members 36 and 38 in turn heat adhesive 14 between FRP members 10 and 12. This bonding technique has the disadvantage of requiring a high cycle time when compared to the bonding techniques illustrated in prior art FIGS. 1 and 2.
When bonded FRP assemblies are used in exterior automotive body applications, it is of the utmost importance that the bonding technique employed does not adversely affect the surface qualities of the exterior FRP members and that the technique provides even bonding notwithstanding the size of the FRP members.
Other previous techniques, such as U.S. Pat. No. 5,554,252, proposed an improved process, however leaving several shortcomings of its own. For instance, conventional methods are unable to get the sheet molding compound (SMC) substrate to an adequately high temperature to provide adequate handling strength to the bonded assembly after 60 to 90 seconds in the bonding fixture. To obtain a lower cycle time more expensive bonding fixtures would be required than what is used with current hot air bonding techniques. In addition, conventional methods have inferior rate of heat transfer that makes the cycle time longer. Much of this inferior heat transfer rate is a result of the use of laminar air flow, which through conventional teachings, provides a better rate of heat transfer than a turbulent air flow system, Methods such as U.S. Pat. No. 5,554,252 may achieve a more desirable cycle time, but the inferior method disclosed would not produce adequate heat transfer rates causing surface deformations along the adhesion bond line of the adhesively joined panel (aka bond-line read-out) because of the use of laminar air flow. Routine experimentation and optimization of U.S. Pat. No. 5,554,252 does not allow for the use of ambient air flow successfully to provide the 60-90 second cure time.
Other short comings of the current state of the art is that using conventional techniques, urethane adhesives cannot be used in SMC bonded assemblies, such as hoods and decklids that have to go thru e-coat (temperatures in excess of 205° C.). Conventional hot air bonding techniques are targeted for use in epoxy adhesives, where rapid cure chemistries (catalysts) are available, while not available for other adhesives such as urethanes. Finally, none of the present state of the art provide for contoured surfaces within the heating assembly that may be adjusted to promote, assist, or optimize air flow for a desired effect.
Thus, it would be desirable to produce a bonding apparatus that improves the surface qualities of a resulting FRP assembly and that provides uniform bonding and handling strength throughout the assembly within a short period of time, such as 60 to 90 seconds, without bond-line read-out. Further, it would be desirable to provide a bonding technique that expeditiously adheres a first FRP member to a reinforcement FRP member without affecting surface qualities or bonding strength characteristics and that have the ability to obtain adequately high temperature for SMC substrates to support these desirable characteristics.